It is well known in the art that the direct liquefaction of the coal is based on hydrogenating treatments, which increase the hydrogen/carbon ratio from 0.7-0.8 to 1 or to values near to 1.
Such processes consist in a partial cracking, under hydrogenating conditions, of the organic structure of the coal. Together with the liquid products gaseous and solid products are formed, their quantities being a function of the operating conditions and of the type of the process.
Generally speaking, the liquefaction process is based on a fundamentally thermal reaction, leading to the formation of radicals, which are stabilized by the hydrogen, such hydrogen having the scope of preventing such radicals from returning back to the form of large, less reactive molecules, and on a catalytic hydrogenation, which reduces the complexity of the molecules by means of the cracking of the bonds between some carbon atoms and other atoms of carbon, oxygen, nitrogen and sulphur.
These two reactions can be effected either as only one stage, or as two separate stages.
The results are however that the more complex ring structures are broken down, in the meanwhile oxygen, nitrogen and sulphur are reduced, or in some appropriate cases eliminated, as water, ammonia, and hydrogen sulphide.
The reactions are carried out in the presence of a solvent, usually resulting from the process itself. Such solvent has an essential function in the conversion, being able to extract the hydrogen-rich products and to dissolve the complex molecules which are formed by the thermal effect and being able to render the reaction with the hydrogen easier, as a transferring and donor agent. The ideal solvent must therefore be characterized by a high solvent power (and therefore by a highly aromatic structure for affinity reasons with the character of the solute) and good properties as a hydrogen donor (and it must therefore be easily susceptible of being hydrogenated as well as of easily transferring to the coal the hydrogen received).
From the liquefaction processes products can be obtained, ranging from refined coal, still being solid at room temperature, with a low content of sulphur and ashes, to light liquid products such as the gasoline. In the first case, the highest energy and weight yields can be obtained; upon increasing the severity of the hydrogenation reaction, leading to increasing rates of the hydrocracking reactions, both these yields decrease.
The trends which have been followed up to now for the liquefaction of the coal to medium/light products can be schematically summarized by the two following process lines:
high severity single stage liquefaction
multi-stage liquefaction, with different severity rate stages.
In the first case, both the thermal reaction and the catalytic reaction takes place in a single reactor, under a compromise condition between the two optimum conditions for the two reactions: a severe hydrocracking is usually obtained, originating distillable products, with notable advantage as for the delicate and expensive separation of the liquid products and the reacted solid products, as such separation can take place in this case by means of vacuum flashing.
A disadvantage is, however, that large quantities of gaseous undesired products are originated, with a resultant high consumption of hydrogen.
By operating according to a multi-stage outline, it is possible to carry out both the thermal and the catalytic reactions under optimum conditions; more particularly, the first liquefaction stage can be effected as a low severity reaction thus realizing the transformation of the coal into a liquid extract, with a low production of gaseous compounds, thanks to the minor importance of the hydrocracking reactions.
In this case, however, since the resulting products are mostly non-distillable products, it is necessary to resort to a more complex procedure for the solid/liquid separation than the vacuum distillation procedure, such as a treatment with an anti-solvent, or a filtration treatment.
Finally, after the solid/liquid separation stage, the extracted products are submitted to a subsequent hydrocracking stage, under controlled catalytic conditions, in order to transform such extracted products into lighter products.
The advantage thus obtained consists altogether in a higher yield of use of the hydrogen supplied, with a lower global consumption rate, and in a higher flexibility of the process, resulting in a larger choice of the range of products which can be possibly obtained.